1. Field of the Invention
The present invention relates to gas sensor units adapted for use in apparatus for controlling an air-to-fuel ratio (hereinafter the “air-fuel ratio”) of engines.
2. Description of Related Art
It is known that, in order to maintain an engine in satisfactory operating condition while reducing hazardous ingredients contained in the exhaust gas from the engine, eg., automobile engines, to comply with the exhaust gas regulations, a gas sensor, such as an O2 sensor, is used for controlling fuel flow to achieve the stoichiometric, air-fuel ratio. Moreover, a three-way catalyst may be used to make the exhaust gas harmless or less harmful.
Nevertheless, if the above-mentioned methods are adopted for controlling the air-fuel ratio by regulating the fuel flow in a carburetor, a control unit with a significant volume may be required, in addition to the O2 sensor and the three-way catalyst. As a result, the engine may increase significantly in size, complexity, weight, and cost. Therefore, it will be difficult to adopt such a system, particularly for a light, which require reduced or limited size and weight.
Consequently, an alternative air-fuel ratio control system may be provided in which the density of a particular hazardous ingredient is detected by using as an exhaust gas sensor. A gas sensor may be used, which is less expensive than the O2 sensor, e.g., a gas sensor for detecting a combustible gas, such as carbon monoxide (hereinafter “CO”), hydrogen, hydrocarbons, or atmospheric gas. Thus, control of the air-fuel ratio may be accomplished by a control unit of relatively low volume, but which is capable of achieving the exhaust gas regulations at reduced cost and without using a three-way catalyst.
Nevertheless, such gas sensors for detecting combustible gas may be susceptible to damage or degradation due to vibration because such sensors have a finely constructed sensing element, such as a platinum coil formed by a fine wire with a diameter of about 10 μm. If such a sensor is positioned adjacent to the vibration source of the engine, the sensor may be damaged or degraded because vibration from the engine may be transmitted directly to the sensing element.
As a technique for protecting the exhaust gas sensor from damage or degradation due to vibration, a system is described in Unexamined Japanese Patent Publication No. 11(1999)-281612, in which a gas sensor has a casing to house therein a sensing element. The outside of the casing is covered by a combined filter-and-shock absorbing part. including a PTFE, hollow fiber structure for absorbing shocks from the outside. Another system is described in Unexamined Japanese Patent Publication No. 2002-71625, in which a frame element is provided between the sensing element of an O2 sensor and a protruding part of a protective insulator so as to absorb shocks and vibration.
These techniques may be effective for preventing or reducing damage to or degradation of a sensing element by the use of a shock absorbing material, which stops shocks and vibration from transmitting directly to the exhaust gas sensor. These techniques, however, appear to be based upon the assumption that the vibrations are relatively weak due to their occurrence at a position to some extent apart from the engine forming the vibration source, or that shocks are transmitted from the outside, rather than upon the assumption that an exhaust gas sensor is disposed at a position adjacent to the engine, such as in a portable work machine, e.g., a chain saw, or an outboard machine. Therefore, these techniques may not protect against vibration or avoid or reduce damage or degradation by sufficiently absorbing vibration, which might transmit directly to the fragile sensing element.